1. Field of the Invention
The invention relates to magnetic bubble devices and more particularly to an adjustable magnetic bias field structure for these magnetic bubble devices.
2. Prior Art
Magnetic domain or bubble devices are well-known in the art. These devices generally comprise a thin film of magnetic material in which small cylindrical areas can be generated and maintained. Domain propagation is along Permalloy tracks laid down on the surface of the film.
A magnetic bubble device requires a permanent magnetic bias field extending normal to the surface of the thin film in order to maintain the domains in a cylindrical form. The bias field must be maintained uniform within a fairly narrow range of about .+-. 1 oersted, especially if a number of magnetic bubble chips are to be mounted on a single substrate as is common in current practice. Generally, the domains in the thin film are propagated along Permalloy tracks by a rotating field generated by a field coil assembly which surrounds the substrates. The substrate-field coil assembly, in turn, is placed within the permanent magnetic bias field structure.
Many magnetic bias field structures have been suggested in the prior art. An early proposal was to use two slabs or plates of magnetically "hard" (low permeability) material (such as barium ferrite) between which a magnetic bubble device would be placed. However, this construction was soon found to have a major drawback in that the magnetic field produced between the two plates is not very uniform since the air gap at the edges of the plates causes distortion of the magnetic field through the well-known "edge-effect" caused by the distortion of the magnetic field lines near the edges of the magnetic plates. Due to the absence of adjacent magnetic dipoles beyond the edges of a magnetic material, field lines will tend to bend slightly outward beyond the edges of the magnetic material. This causes the number of field lines per unit area (flux) to be slightly lower near the edges. Consequently, the magnetic field between two magnetic plates tends to be non-homogeneous near the air gap between the edges of the plates.
It has been recognized that a bias structure utilizing what is known as a "Watson" magnet will give substantially better field uniformity over a larger area than the structure previously described. In the Watson magnet configuration two magnetically "hard" bar magnets are placed between two magnetically "soft," highly permeable plates (such as soft ferrite or Permalloy) to provide a uniform magnetic field between the plates. Such a structure is described in "Permanent Magnet Bias Schemes for Bubble Memory Applications," William Lyons, IBM Technical Report TR 22.1633, 8 May 1973, pp. 1-12. A variation of this technique, shown in IBM Tech. Discl. Bull. Vol. 16, No. 7, Dec. 1973, pp. 2129-30, uses a number of cylindrical slugs instead of bar magnets to produce the required bias field with excellent homogeneity. Since in the above-described devices the magnetically "hard" bar magnets or slugs are in contact with the magnetically "soft" plates near their edges, a continuous and uninterrupted path is provided for the magnetic field between the two plates near their edges. As a consequence edge effects are reduced and field uniformity is improved over a larger portion of the plates.
Usually, the bias structure is first assembled and then magnetized to the required value by use of a magnet charger. It is also known to use a succession of two oppositely directed external magnetic fields to adjust a fixed magnetic biasing structure, as is shown in U.S. Pat. No. 3,931,618. However, because the bias field sometimes varies slightly from package to package, a method for mechanically "fine tuning" or adjustment of the field is required.
Various adjustment schemes for magnetic bias structures have been proposed in the prior art. One such technique is shown in U.S. Pat. No. 3,927,397, in which a pair of parallel facing magnetically permeable plates, each having a ferromagentic slab laminated thereto, are adjustably spaced apart by four threaded non-magnetic bolts. The non-magnetic bolts are adjustable by means of a slotted head provided at one end of each bolt. A magnetic bubble device would be placed between the parallel plates. This structure suffers from the disadvantages that a pair of specially laminated plate structures must be used and, since an air gap is still present between the two parallel facing plates, edge effects produce a non-homogeneous magnetic field near the outer edges of the biasing structure.
Another adjustable magnetic biasing structure is shown in U.S. Pat. No. 3,711,841. A pair of parallel facing plates of high magnetic permeability are separated by a pair of bar magnets in the familiar Watson type magnet arrangement. A magnetic bubble device is placed within the volume defined by the two plates and the two bar magnets. The separation between the plates, and hence the strength of the magnetic field, is adjustable by means of four non-magnetic threaded bolts. While this structure reduces edge effects somewhat by allowing a continuous path for the magnetic field through the plates and contacting bar magnets, this structure suffers from two disadvantages: first, this structure requires a pair of relatively "hard" (low permeability) bar magnets to provide the return path for the magnetic field near the edges; and second, the act of adjusting the air gap by means of the non-magnetic bolts may cause a portion of one of the magnetic plates to lift away from a portion of a bar magnet and as a consequence will produce a distortion of the flow of the magnetic field. This will cause a non-uniform field to be produced in the vicinity of the air gap resulting in degradation of magnetic bubble device performance.
An arrangement similar to the mechanically adjustable air-gap Watson-type magnetic biasing structure shown in U.S. Pat. No. 3,711,841, is described in "Magnetic Bubble Mass Memory," Michaelis and Bonyhard, IEEE Transactions on Magnetics, Vol. MAG-9, No. 3, Sept. 1973, pp. 436-440. This structure suffers from a similar disadvantage in that non-magnetic screw adjustment of the plate separation will cause magnetic field distortions along portions of the plates which are moved out of contact with the pair of bar magnets.